The present invention is related to the field of image projection.
A number of projection systems designed to render images, or more specifically, an image frame, by successively turning on and off selected ones of a number of light emitting devices have been proposed. Typically, each of the light emitting devices emits light in a primary color, e.g. red, green or blue. Typically, each of the light emitting devices is driven in accordance with its drive current design point, which is often selected to enable the light emitting device to have a reasonable chance of having a reasonably lengthy operational life.
Additionally, many of these projection systems employ light valves to control the color of each pixel of the image frame being rendered. A light valve e.g. may be a micro-mirror device, (for example DLP® device from Texas Instruments, Dallas, Tex.) and the color of a pixel may be controlled by controlling the on or off state of one or more micro-mirrors of the micro-mirror device through pulse width modulation of the micro-mirror device in synchronization with colored illumination light. Due to the responsiveness (response time) limits of some of the micro-mirrors (in the light valves), many of these projection systems have difficulty effectively imaging dark scenes, where discernment between the lowest gray levels are desired. An example is night scenes in a movie. Typically, temporal and/or spatial dithering is required. Temporal dithering refers to the effectuating of a color (and grayscale) effect through color controls that span different frame periods, whereas spatial dithering refers to the effectuating of a color (and grayscale) effect through color controls that span different pixels. Practice of either or both of these dithering techniques adds to operational complexity and cost (since a high compute power image processing ASIC is required to implement spatial and/or temporal dithering). In addition, these dithering techniques result in image artifacts: the appearance of noise or “snow” or lack of image detail in darker images and blurring of moving objects.
Another problem suffered by non-CRT projection systems is their inability to provide “peak white” to a small area of an image frame like CRT displays can.